Writing Lab Reports for ASTR 110L

While many of you already have a good idea of the role of Lab Reports in the physical sciences, perhaps for some of you it is the first time in a long time (or the first time ever) that you have had to write one.

We will try to be concrete about our minimum expectations for content of any given lab report: at the end of the handout for each project or experiment, we specify the basic measurements, observations, calculations, and/or questions we want you to include. We do not dictate a completely rigid report format, since we want you to develop personal judgment about what is important and useful to report, and to give you the flexibility to shape your own report accordingly. However, there is still a general format that most science lab reports are expected to follow, and we do have a few common expectations of completeness and readability for everyone's work.

Below are some guidelines for Lab Reports in our course. We are not necessarily looking for long reports, just completeness (inclusion of all scientifically important details) and clarity (good organization, and exclusion of scientifically irrelevant items). Approach your report so that after reading it, an intelligent person would understand the purpose of your observation or experiment (what you wanted to measure or demonstrate), the general steps that were involved, what your outcome was, and whether you were successful. Please see your instructor with any questions.

0. TITLE

Include a title! It does not need to be anything fancy or original; just use something simple that states the report's topic, like "Motions of Mars" or "Parallax Lab." It is just something to distinguish the report at a glance from the other lab reports you will be writing.

1. INTRODUCTION & OBJECTIVE

Briefly describe the phenomenon that we are studying, and why or how it occurs. Also, paint the bigger picture: why is this useful or important in astronomy?

After providing some background, give the reader a concrete idea what you were trying to observe or measure. What is the ultimate goal or objective of your experiment or project? To measure a distance? To find the focal length of a lens? To determine the type of variable star you are looking at, or its period of variability? This will be the "punch line" of the whole report; it is the scientific question that you are hoping to answer by performing the experiment.

Omit things that are irrelevant to the project's measurements or calculation. For example, bad weather might be the reason that we perform an indoor experiment like the Simple Telescope or Parallax lab, but that does NOT need to go into the lab report's introduction - it has nothing to do with the lab's purpose or measurements. (You can note it in your observing log, where you would normally record the night's observing conditions.)

2. PROCEDURE & EQUIPMENT

Give a quick verbal sketch of the procedure you used, and if it helps, include one or more diagrams. You should not simply regurgitate the entire lab handout, nor do you need to describe every last detail of your procedure (like "I removed the lens caps from my binoculars"). Instead, use your own words to summarize the highlights and important elements.

Set the stage for the observations or measurements to come in the next section by briefly describing for the reader the tools you used. Specify the equipment with which you made your measurements: naked eye, binoculars, telescope, cross-staff, spectroscope, etc. (Why do you think this is important?) For example, what style of telescope was used? How large was the mirror? Which eyepiece was used? For astronomical observations, include the location where you made your measurements. (Why might this matter for some observations?)

Suppose that you use a cross-staff to measure some angular separations between stars. Since a cross-staff is not an everyday object, you could provide a brief description; a simple diagram would be even better. Listing the cross staff's materials (wood, pushpin, paper clips) is not as important as describing the purpose of its parts: a ruler attached perpendicularly to a rod, designed to suspend the ruler always at a fixed distance in front of the observer's eye. Then summarize the cross-staff's function: (1) a cross staff tells the observer holding it the angular separation between two distant objects; and (2) 1 cm on our particular cross staff rulers corresponds to 1° of angular separation between the two objects being sighted. This second detail is especially important if the data you recorded were the number of centimeters you measured on the cross staff - the reader needs to know how you plan to convert those centimeters into degrees.

Omit obvious items like "pen," "paper," "calculator," and "observing log" from your list of equipment - those are assumed.

3. RESULTS  (MEASUREMENTS & CALCULATIONS)

Report (1) the measurements or observations you made, and (2) any calculations or results you derive from them. When reporting a large number of data (say, the brightness of a variable star as measured on 15 different occasions), present your data in a table. If you took multiple measurements of the same value (such as the Moon's diameter) that you intended to average together, show all of your recorded values in a table, then give their average at the end. (Why do you think this is a good technique?)

What are the units of your physical values? feet? inches? meters? degrees? It is vital that you include units on any measurements or calculated values.

If you use a formula to make a calculation, always present the initial formula with just the plain variables first. For example, in your Parallax lab report, first write: D = (1/2) * (360° / theta) * b before showing calculations with any of your measured data for theta or b. Never report just your calculated value for D with no indication of the formula used. (Many of our labs do not involve formulas, but this is essential for those that do.) Once you have given your initial formula, it is not necessary to show every gory algebraic step of every calculation. In your lab report, omit obvious arithmetic and algebraic steps. When you need to perform the same calculation over and over for different values of the same variable, just one "sample calculation" is sufficient.

Report calculated values with an appropriate number of significant figures. Never write every digit that appears on your calculator screen in your final lab report - round off your value at a decimal place which is appropriate for the precision/uncertainty of that particular value. For example, if one of your calculations yields an answer of 66.666666666666 meters, you should probably report the value as "67 m" or "66.7 m". If you have further questions about this, please see your instructor.

A good scientist doesn't just report a measured value without also including how precisely that value was measured. This is called a value's uncertainty or error, and simply tells the reader how closely you were able to measure that particular physical value. When measuring a length, were you able to measure it to the nearest millimeter? the nearest centimeter? or just the nearest 10 cm? When measuring a time interval, were you timing to the nearest minute? the nearest second? the nearest 0.1 second? Your sources of error might be due to limitations in your measuring devices (such as a tape measure whose marks are only shown to the nearest centimeter), or might have random or human origins (such as your shakiness in holding up your cross-staff preventing you from measuring the separation between two objects any better than the nearest 0.5°). Either way, it is always good to record a value's uncertainty along with the value itself, first in your observing log, and then summarized or discussed alongside your reported data in your final lab report. A larger discussion of how your sources of error might have affected your final outcome belongs in your conclusion (see below).

4. CONCLUSIONS

Briefly summarize your final results from the previous section, and then assess how well your results agree with what you expected to find. (In a scientific paper, this would be a discussion of whether the predictions of the "scientific hypothesis" laid out at the start of the paper are supported or refuted by the results of the experiment.) For an experiment with quantitative results, your results can be said to "agree with" your expected outcome if the two are the same to within the uncertainty (or "error") of your results. For our lab experiments with non-numerical, qualitative results, you should still discuss whether your actual results conform reasonably with our expected/predicted outcome. In either case, if your results differ from what was expected, look back over your data, methods, and calculations, and try to identify the cause of the difference. Which of your sources of error is most likely responsible for the difference? Or if your outcome is grossly unexpected, can you identify one of your steps that went wrong?

For example, here are some possible sources of error in the Parallax Lab:
(1) You might estimate that your cross-staff limited your precision in measuring angles to only the nearest 0.5° or so, due to how steadily you could hold the cross-staff, and how precisely you could read the ruler. Being off by 0.5° results in a big miscalculation of D when theta is small, or a smaller miscalculation of D when theta is big - you can find out exactly how bad it is for your particular case by calculating D for your measured theta (say 8.0°), then plugging in 7.5° and 8.5° instead and seeing how much your answer for D varies. This is an important scientific technique for estimating the uncertainty in your final (calculated) result, given the size of the uncertainty in your measured data.
(2) If your background reference object was too close, or (3) if your angle theta was too large, our parallax formula becomes increasingly inaccurate - the formula is only an approximation which works best for small values of theta.
(4) If you measured your baseline length sloppily, or (5) if you might have leaned your body to one side while taking your measurement, either one would have introduced error into your measurement for b. How many inches would you estimate your error in b to be? How does this affect your result for D?
(6) If you measured/calculated a value that was way off, can you figure out what might have happened?

Your "ability to use the formula" or "ability to use my calculator" are NOT "sources of error" in the same sense as experimental/observational sources of error... at least they shouldn't be! Get help with math or with using your calculator from one of your instructors or from a friend.

Many of our lab exercises are more qualitative than quantitative, in which case your error discussion will most likely be qualitative and brief.

Personal thoughts, such as whether you liked or disliked the lab, or would recommend it to friends, or thought it took too long, are generally not part of a scientific conclusion. While we do welcome your feedback on our lab projects, it is most appropriate to do so outside of your lab reports.

THROUGHOUT your report

A diagram or sketch is worth a thousand words! For example, the Parallax lab lends itself naturally to a top-view diagram of the parallactic triangle, showing your target object and baseline being measured. Feel free to draw your diagram in your report with a pencil/pen and ruler; not everything needs to be a fancy computer graphic, even if you are using your computer for the rest of the text. Be sure to label any diagram well, so that the physical quantities discussed in the text of your report (like theta, b, and D) become immediately clear to the reader.

Sanity-check your results. Does a final answer of 10 miles make sense when calculating the distance to your roommate's desk? If not, then recheck your calculations or your original data/measurements. Sometimes you will have the chance to take a measurement multiple times, or in more than one way... do it, then compare or average your results. Do they turn out the same? (Why do you think reproduceability of outcomes or of results is so important in science?)

Proofread your report! (It is highly recommended to use a word processor, since it makes correcting mistakes much easier than writing out the report by hand.) However, feel free to hand-write formulas and hand-draw diagrams, since most word processors are not formula-friendly. Double spacing or leaving wide margins makes it much easier for your instructor to write in comments. Finally, check your spelling and grammar.

The passive voice is frequently employed in scientific papers, and you are welcome to use it in your lab reports. While English teachers almost always prefer active sentences in creative writing ("I observed the brightness of the star"), scientific reports usually leave out the observer as the subject, using the passive voice instead ("The brightness of the star was observed").